1. Field of the Invention
This invention relates to a fuse furnished with a fuse element having a pair of terminal connection portions and a fusible member, and a method of producing the same.
The fuse of the invention is used especially for protecting electric circuits of automobiles.
2. Related Art
Explanation will be made to a conventional fuse by one example of a fusible link of a cartridge type. The fusible link 201 shown in FIGS. 14A and 14B is composed of a synthetic resin-made case 202 and a metal-made fuse element 203 carried within the case 202. The fuse element 203 has a pair of terminal connection portions 204, 204 and a fusible member 205 for electrically connecting the terminal connection portions 204, 204 each other, and at a central part of the fusible member 205, a fusion-breaking portion 206 is formed for being fused and broken when an overload electrical current flows.
FIGS. 15A through 15C are developing views of the fuse element 203. The fuse element 203 is formed by punching a thin metal sheet having electric conductivity in a shape as the developed fuse element 207 (207a, 207b, 207c) shown in FIGS. 15A through 15C, and then subjecting to a bending step. The fusion-breaking portion 206 is shaped to have different widths W for forming necessary cross sectional areas. For example, FIG. 15A shows an example that fusion-breaking electric current is 30 A (ampere), FIG. 15B shows another example of 40 A, and FIG. 15C is 50 A. The widths W of the fusion-breaking portion 206 is determined to be W1<W2<W3.
Incidentally, in the above mentioned related art, for determining the fusion-breaking electric current, the width W of the fusion-breaking portion 206 was necessary to change, and sorts of the developed fuse elements 207 were present by the number of determining the fusion-breaking electric current. Therefore, metal molds were required in response to the sorts of the developed fuse elements 207, and production costs were influenced thereby.
If plural kinds of developed fuse elements 207 are taken out in the same metal mold, problems occur that it is difficult to enlarge the metal mold or adjust production of single kind of developed fuse elements.
On the other hands, as fuses for protecting electric circuits passing transient current of conductive rate being up to around 200% such as motor load circuits of automobiles, fusible links (F/L) have conventionally been used. The fusible link is demanded to usefully function to protect circuits when occurs burst current of conductive rate being more than 200% at time of such as dead short. That is, where the conductive current is twice of a rated value (conductive rate is 200%), such current is determined to be a boundary value, and in case, a larger current area than it is classified to be a dead short area and a lower area that it is classified to be a rare short, demanded are such fuses having characteristics useful respectively in the dead short area and the rare short area.
To state in more detail, when passing the large transient current as the dead short time, a circuit is necessarily cut off prior to breakage of a load circuit, fusion-breaking of a lead wire connected to the load circuit, or fuming occurrence. Further, for example, when opening or closing a power window of the vehicle door, a motor lock current in a middle current area of the conductive rate being less than 200% flows during about 10 seconds, and even if the motor lock current frequently flows, the circuit must not be cut off.
FIG. 16 shows a fuse element of a fuse having a delay-breaking characteristic disclosed in JP-A-5-166453. The fuse element 211 is composed of a pair of opposite terminal connection portions 212 and a fusible member 215 furnished at an intermediate part of the pair of terminal connection portions 212 and securing metal chips 213 with wrapping parts 214. The metal chip is a wire material formed by forcing out a low melting point metal and cutting it out, while the fusible member 215 is formed of a plate-like fusible metal conductor.
As to the quality of the fusible member 215, a basic material thereof is the same Cu alloy as a conductive wire, and a cross sectional area is reduced in size for instantaneously breaking when a large current flows. On the other hand, the quality of the metal chip 213 is Sn having a lower melting point than that of Cu, so that it is fused by a temperature heightening owing to electric conduction, and is dispersed within the fusible member 215 to form an alloy phase. Accordingly, at the middle or small current areas, the metal chip 213 is fused and broken by the alloy phase of higher resistance than the basic Cu alloy.
In regard to the fuse element 211 of the existing fuse, at a step of setting up the metal chips 213 on the fusible member 215, since the metal chips 213 have to be cut out at a predetermined length and a caulking is required, there is a problem that a dimensional management of the metal chips 213 is not easy. Besides, dimensions of the metal chips 213 are varied by number of setting the fusing electric current, and so another problem is that a plurality of caulking molds are required.
FIG. 17 shows a fuse element of a fuse disclosed in JP-A-8-17328. The fuse element 216 was invented for solving the above problems, and the fusible member 217 of the fuse element 216 is secured with the metal chip 219 made of a low melting point metal having a hollow portion 218 by the wrapping part 220. The metal chip 219 is formed fixedly at an outside, and if changing a diameter of a piercing hole as the hollow portion 218, a fusion-breaking characteristic of the fusible member 217 is able to be adjusted.
However, although the dimensional management has been easier than that of the metal chip 215 (see FIG. 16), there still remains a problem that the dimensional management is not yet sufficiently easy when producing. That the dimensional management is not sufficient, has a problem that the fusion-breaking time is brought about with dispersion.
The above mentioned two existing examples require the caulking for fixing metal chips, so that there is probability of creating inconvenience as deformation by the caulking step, inevitably causing cost-up thereby.